1. Field
The embodiments described below relate generally to the delivery of radiation to a patient. More specifically, some embodiments are directed to determining a spatial dose distribution based on a three-dimensional image generated using a megavoltage radiation beam.
2. Description
According to conventional radiation therapy, a beam of radiation is directed toward a tumor located within a patient. The radiation beam delivers a predetermined dose of therapeutic radiation to the tumor according to an established treatment plan. The delivered radiation kills cells of the tumor by causing ionizations within the cells.
Recent advances in fractionated external beam radiation therapy, such as three-dimensional conformal and intensity-modulated radiation therapy (IMRT), have increased the ability to deliver radiation doses that conform tightly to a target volume. This tight conformance results in steep dose gradients inside the volume. For example, IMRT can create a dose gradient of 10% mm−1 inside a target volume.
A treatment plan is designed assuming that a target volume will be in a particular position relative to a beam source during treatment. If the relevant portions are not positioned exactly as required by the treatment plan, the steep gradient may occur within sensitive healthy tissue surrounding the volume. Thus, it is increasingly important to precisely position the target volume with respect to the beam source.
Currently, internal bony markers, external markers and patient-immobilizing masks and casts are used to reproduce a desired skeletal position of the patient with respect to the beam source. However, the effectiveness of these alignment and immobilization techniques is limited by changes in the tumor location (as well as the locations of adjacent internal organs) relative to the markers. For example, the prostate can shift up to 1 cm relative to the pelvic bones due to variations in rectal/bladder filling. A tumor may shrink and the patient may lose significant weight, also causing the tumor to shift with respect to internal and external markers.
Portal images, which are projection images of the treatment field within the patient, are currently used to confirm the patient position and verify proper irradiation of the tumor. An electronic portal imaging device (EPID) may acquire a digital portal image that is used to adjust the patient position before each daily treatment. For example, using implanted gold markers to locate the prostate, daily portal imaging has been used to position the prostate with 1-2 mm accuracy.
The effectiveness of portal image-based positioning is limited, however, because implanted markers are needed to visualize soft tissue and because the full three-dimensional volume to which radiation will be delivered is inadequately represented by a two-dimensional portal image. Therefore, considerable research now focuses on providing three-dimensional imaging of the patient immediately prior to treatment delivery (i.e., when the patient is on the treatment table). Systems attempting to provide such imaging include: (1) a “CT on rails” system, requiring an additional diagnostic computed tomography machine in the treatment room; (2) a kilovoltage cone beam CT (kVCBCT) system, consisting of an additional kilovoltage X-ray source and detector attached to a treatment gantry; (3) a megavoltage cone beam CT (MVCBCT) system using the pre-existing treatment machine and an EPID for imaging; (4) a MVCT system, using the pre-existing treatment machine with an attached arc of detectors; and (5) a tomotherapy system, replacing the traditional treatment machine with a CT ring and a MV beam source.
An IGRT system may use any of the foregoing imaging modalities to translate and rotate a patient to a position required by a treatment plan. These imaging modalities may also be used to modify radiation delivery to account for changing relative positions of internal organs and changing shapes of the organs between treatment fractions. In this case, a pre-treatment image may be acquired and the treatment beam may be adjusted based thereon immediately before irradiation.
Images acquired in the treatment room may also be used to adjust the treatment beam in situations where organs are expected to move significantly during treatment. For these situations, beam delivery may be halted when the target volume is out of a certain acceptable region, or the beam may track the target volume during irradiation using specially-designed mobile linear accelerators.
If the dose that was delivered in previous fractions can be estimated, the treatment plan for future fractions may be modified to compensate for prior dosimetric errors. This “dose-guided radiation therapy” (DGRT) could correct for errors due to patient anatomical changes as well as machine delivery errors. However, current methods to determine delivered or to-be-delivered doses are inefficient and/or inaccurate.
According to some current methods, diodes or thermoluminescent dosimeters are placed on the patient surface and/or implantable MOSFET dosimeters are embedded within critical internal structures. The time and effort required to place or implant such devices limits their use during treatment. More importantly, these techniques provide only point dose measurements, which are not suitable for determining dose distributions based on which future fractions may be modified.
What is needed is an efficient system to determine a three-dimensional dose distribution. Such a method may detect dosimetric errors produced by machine delivery errors, patient positioning errors, and/or variations over time in organ positions and shapes.